FIG. 1 illustrates the performance-degrading phenomena of DVR on the bulk electric grid, by showing a Fault-Induced Delayed Voltage Recovery (“FIDVR”) following a 230-kV transmission fault in Southern California. The x axis is time and the y axis is voltage. Section 100 of FIG. 1 depicts the steady state voltage. At time 102, a fault occurs and clears on the grid. A fault is a short circuit on the utility line caused by, for example, a tree falling onto the line or a lightning strike, or any other event that would cause a short circuit in the utility line. The high currents associated with faults can cause a substantial reduction in line voltage due to the basic relationship of V=IR, or voltage=current times resistance. During the fault, the utility line voltage may drop by half or more. The duration of faults on utility transmission and distribution equipment is typically very short 4-10 cycles, or fraction of a second, before the faulted equipment is isolated through the opening of breakers. Normally, voltage at the grid level should immediately return to pre-fault normal levels as soon as the faulted equipment is opened/isolated and fault current ceases to flow.
However, if the system is not operating normally, during the brief second that the fault is occurring, a motor, for example, an air conditioner, may sense the large voltage drop and stall. Some air conditioners disconnect from the grid based on voltage drops, while others only will disconnect when a thermal sensor detects an elevated operating temperature. Therefore, this second set of air conditioners will not disconnect at the voltage drop caused by a fault. Instead, because of the drop in voltage, the air conditioner may stall and draw 6-8 times more current than when in normal operation. The large current draw causes the air conditioner to produce excess heat. These air conditioners may not disconnect until the thermal sensor detects a requisite level of heat, which may be 20-30 seconds after the fault has occurred. The stalled air conditioners also may cause other thermal sensor air conditioners to stall. This can build upon itself into a type of blackout called a voltage collapse. Therefore, these thermal condition triggered air conditioners exacerbate the fault and create a DVR condition. Because of the growing number of residential air conditioners with thermal sensors, DVR conditions have become a larger problem.
Referring back to FIG. 1, time period 104 is the delayed voltage recovery period during which some loads (e.g. motors, air conditioners, etc.) may stall and start to disconnect from the grid via thermal protection switches. Typically, period 104 can extend for about 20 seconds. Then, at time 106, a power overshoot typically occurs, caused by line and substation voltage-support capacitors remaining on line. At time 108, the capacitors disconnect because of the overshoot in voltage at time 106. At time 110, the loads that had previously disconnected come back online. At time 112, the grid is under voltage for a time period while the capacitors that turned off at time 108 remain off. This under voltage period increases the likelihood of another delayed voltage recovery event. Taken together the recovery time, that is, the time it takes the system to return to steady state conditions at 100 after fault 102 can last up to approximately 30 seconds. As shown in FIG. 1, the voltage deviation on the grid during a delayed voltage recovery event can be extreme.
The first recorded DVR event was in the Tennessee Valley Authority transmission system on Aug. 22, 1987. Present systems have been unable to detect DVRs as they occur; most DVRs are detected long after the fact by reviewing power grid data.
Known prior-deployed solutions have included combustion turbine driven electric generators and Static VAR Compensators, or SVC's. However, generators burn fossil fuel, thereby wasting resources and adding green house gasses, and some generators do not have response times adequate (fast enough) to prevent a voltage collapse. SVC's have declining volt-amperes reactive (“VAR”) output versus terminal voltage. Thus, when VAR support is needed most, these solutions have reduced output capacity and effectiveness. Currently, there are limited power industry options and solutions to this relatively new/emerging risk to the U.S. electric power system.